[css-values] Fix w3c#179 by adding links to serialization of CSSOM.

Author: fantasai

css-values/Overview.bs

@@ -604,7 +604,8 @@ Fragment URLs</h5>
 	This reference must always be treated as same-document
 	(rather than cross-document).
 
-	When serializing a ''url()'' with the <a>local url flag</a> set,
+	When <a href="https://www.w3.org/TR/cssom-1/#serializing-css-values">serializing</a>
+	a ''url()'' with the <a>local url flag</a> set,
 	it must serialize as just the fragment.
 
 	<details class=note>
@@ -811,7 +812,7 @@ Numbers with Units: <a>dimensions</a></h3>
 <h3 id="compat">
 Compatible Units</h3>
 
-	When serializing <a>computed values</a>,
+	When <a href="https://www.w3.org/TR/cssom-1/#serializing-css-values">serializing</a> <a>computed values</a> [[!CSSOM]],
 	<dfn export local-lt=compatible>compatible units</dfn>
 	(those related by a static multiplicative factor,
 	like the 96:1 factor between ''px'' and ''in'',
@@ -1739,6 +1740,8 @@ Serialization</h4>
 		which then drops the ''calc()'' wrapper.)
 	</div>
 
+	See [[!CSSOM]] for further information on serialization.
+
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